Existing Technology and Problems Encountered
Focused Ion Beam (FIB) tools produce a focused, energetic beam of positively charged ions (subsequently referred to as the primary ion beam or primary ions), which can be used to image or alter microscopic structures. When the primary ion beam is scanned over the surface of the sample, an assortment of secondary particles is sputtered from the surface: electrons, ions, and neutrals.
In commercial FIB tools, a portion of the charged secondary particles is diverted to a detector, usually a microchannel plate (MCP) or scintillator-type detector, where the signal is amplified and measured. To create an image, the measured secondary signal intensity is converted to a grey-scale color and displayed on a screen.
When an MCP detector is used, two imaging modes are generally available, depending on which secondary particle is being detected. In electron mode a positive voltage is applied to the MCP to attract secondary electrons (secondary ions are repelled by the MCP and are not collected). In ion mode, the MCP is biased negative to collect secondary ions (secondary electrons are repelled by the MCP and are not collected).
FIG. 1 illustrates the operation of a typical FIB tool in electron mode. FIB tool 10, schematically representing the ion source, focusing optics and deflection coils that scan the beam over workpiece (or sample) 5, generates ion beam 50. Beam 50 strikes a point on the workpiece and generates electrons 14 and secondary ions 12.
The detector 20 is biased positive with respect to the workpiece by bias source 22 to attract electrons (and thereby repel the secondary ions). Detector 20 may be a conventional microchannel plate or any other charged particle detection system. Workpiece 5 is optionally biased at a convenient value.
On the left of the figure, electron source 55 is available to generate electrons that can neutralize charge buildup on the workpiece. This is not used during electron mode.
FIG. 2 illustrates the other standard mode, ion mode, in which detector 20 is biased negative with respect to the workpiece, so that secondary ions are attracted to the detector.
The beam consists of positively charged ions, and therefore a net positive charge accumulates on the workpiece during FIB processing. The buildup of positive charge at various locations within the workpiece can result in damage due to electrostatic discharge (ESD). To prevent ESD damage, a current of electrons may be directed at the workpiece to neutralize such a buildup. In this case, electron source 55, referred to as a flood gun, directs a flood of electrons at workpiece 5 to neutralize charge buildup.
Ion mode has some special advantages over electron mode. It typically has better sensitivity to differences in material composition compared to electron mode. Additionally, ion mode can be used while simultaneously exposing the sample surface to a flood of electrons to neutralize the build-up of positive charge from the primary beam. Electron mode, by contrast, cannot be used during charge neutralization because the flood electrons would be attracted to the MCP and would swamp the secondary electron signal.
For samples that could be damaged by charge build-up from the primary beam, ion mode is essential.
One serious limitation of ion mode, however, is total signal intensity. The number of secondary ions generated by the primary beam is significantly lower than the number of secondary electrons, so the signal-to-noise ratio of ion mode is poor compared to electron mode. In particular, when the primary beam is small (below about 20 pA), the number of secondary ions is so low that ion mode becomes impractical.
Nevertheless, in the semiconductor industry there is growing concern over beam-induced charge damage. Modern integrated circuits (ICs) have smaller and more delicate features, new types of construction (such as SOI), and new materials (such as low-k dielectrics). All of these factors make modern ICs more susceptible to charge damage, so it is necessary to keep beam currents low and use charge neutralization during FIB processing.
Clearly, some means of improving the existing ion detection scheme would enhance the capabilities of the FIB technique.